1. Field of the Invention
This invention relates to digital processing systems and more specifically to digital processing systems requiring multiple internal timing cycle operations.
2. Description of the Prior Art
Digital processing systems include external oscillators that providing timing signals to govern the internal clocks of the central processing unit. Problems arise when the timing of the central processing unit is different from the timing of a peripheral device that is connected to the central processing unit. One solution is to slow the faster device to the speed of the slower device by using the slower device oscillator to govern the internal timing of all devices. This solution is inefficient when using a high speed central processing unit, since the central processing unit will not be allowed to take full advantage of its speed capability. Another solution is to use external flip-flops to switch between different oscillators that are connected to different peripherals in order to slow the central processing down. Two basic problems arise from this solution. The first is the synchronization problem in turning off one oscillator and turning on another ascillator and second, the amount of space required to implement the flip-flop approach.
It is the object of the present invention to provide a switching means that allows an external control device or the central processing unit itself to change the internal timing of the central processing unit to match the internal timing of external peripheral devices that are provided by a separate oscillator.